1. Field of the Invention
The invention concerns a device for clamping and releasing a tool having a tool shaft, the device including a tool receptacle having on its free end an open socket part of an electrically conductive material for receiving the tool shaft with frictional engagement, and with a ring or cylindrical coil shaped induction coil surrounding the socket part of the tool receptacle, which coil can be energized with a preferably high frequency alternating current.
2. Description of the Related Art
Devices of this type are intended to be used for example for clamping or releasing a tool in the form of a milling cutter or drill in the tool receptacle. For this, the tool receptacle is heated in the area of the socket part with the aid of the induction coil, so that the borehole of the socket part expands. The tool is then introduced via its shaft into the heating-enlarged receptacle. During the subsequent cooling of the socket part, the tool shaft is frictionally held in the borehole of the socket part which shrinks as it cools. The diameter of the tool shaft and the socket part are therein so selected, that upon cooling a form-fitting connection results which is fixed against rotation, and which is not released even due to centrifugal forces during rapid rotation. For de-tensioning, the socket part is warmed again via the induction coil, until the tool can be pulled out of the receptacle. The de-tensioning is only possible because the heating spreads from outside towards inwards, so that first the socket part is warmed, prior to the warmth reaching the tensioned tool part. Thereby it is achieved, that first the socket segment expands, so that the still cooler tool can be released out of the receptacle during de-tensioning. This however functions only when the employed tool has low thermal coefficient of expansion and/or low electrical conductivity, for example hardened metal or ceramic. In the case of employment of insertion tools made of tool steel, there are however always problems during de-tensioning.
The invention is thus concerned with the task of further developing the known device of the above described type in such a manner that even tools with a large thermal coefficient of expansion and/or comprised of electrically conductive material can reliably be clamped and released with the aid of induction heating.
The inventive solution is based upon the recognition, that tools of electrically conductive material are heated so rapidly by the not insignificant field of the conventional induction coil in the vicinity of the clamping area, so that the de-tensioning is made difficult or impossible. In order to prevent this, in accordance with the invention it is proposed to reduce the electromagnetic fields in the area of the free end of the socket part to such an extent that a heating of the tool situated in the tool receptacle is prevented. In accordance with the invention this is achieved thereby, that the induction coil is covered over by a pole shoe on its end surface adjacent the free end of the socket part, the pole shoe exhibiting a central opening for the tool and comprised of a magnetically conductive and electrically non-conductive material. By this means it is achieved, that the magnetic field lines are concentrated in the pole shoe on the concerned surface of the induction coil, so that also the part of the tool extending beyond the tool receptacle is effectively shielded against electromagnetic flux or leakage fields.
A particularly effective shielding, is achieved thereby, that the pole shoe partially overlaps the free end of the socket part and lies axially and/or radially against the free end of the socket part. The through-hole opening in the pole shoe exhibits a greater dimension than the tool diameter, so that an air gap with high magnetic resistance exists in the direction of the tool. A further improvement in this respect is achieved thereby, that the pole shoe exhibits a cross-section which narrows partially in the direction towards the opening.
The pole shoe can be formed as a ring disk, which exhibits a concentrically conical fluting or groove towards the opening. Basically it is also possible to form the pole shoe of multiple, spoke-like arranged radial segments.
In order to achieve a rapid as possible warming of the socket part with as low as possible electrical consumption, it is proposed in accordance with a further preferred embodiment of the invention, that the induction coil further includes on its surface opposite to the pole shoe and/or on its outer circumference surface supplementally a magnetic shield of magnetic conductive and electrically non-conductive material. The face of the magnetic shield can be formed as a ring disk with a through hole for the tool receptacle of the socket part. Alternatively to this, the face of the magnetic shield can also be formed of multiple spoke-arranged radial segments. The outer circumference or casing outer-side shield is preferably formed as a cylindrical cage, which is either closed in the circumference direction or can be comprised of multiple axially parallel segments arranged spaced apart from each other in the circumference direction.
For better centering of the induction coil with respect to the socket part it is of advantage, when the pole shoe includes a concentric, ring shaped sintering shoulder for receiving the free end of the socket part and/or for supporting the ring opening of the induction coil.
In order to keep the current strength as low as possible for a pre-determined amount of heat production, it is proposed in accordance with a further preferred embodiment of the invention, that the induction coil includes a preferably multi-layer wound coil winding of a high frequency stranded conductor. It has found that, in this case, the otherwise conventional water cooling of the coil winding can be dispensed with. Since the heating process only requires a few seconds, generally convection cooling via the ambient air suffices. At higher power settings the coil winding can be cooled as necessary by forced air.
The induction coil preferably includes a coil body or bobbin of a ceramic material, which can in addition be used as carrier for the pole shoe and/or the magnetic shield.
Tests have shown that the outer circumference part is heated less rapidly in the vicinity of its free end than in the central area. In order to avoid gap problems, it is thus of advantage, when the socket part includes a registry or fitting borehole for receiving the tool shaft, which is in communication towards its free with a segment of larger diameter. The socket part can exhibit a cylindrical or barrel shaped outer surface.
The tool receptacle could on the one hand be provided directly on a rapidly rotating machine spindle. Alternatively thereto, the tool receptacle can carry a coupling element which carries the socket part and is connectable with a rapidly rotating machine spindle.
The tool receptacle and the induction coil are associated in the above-described orientation for the purpose of coupling and de-tensioning procedures. In order to make possible a simple operation, it is of advantage when the tool receptacle and the induction coil are moveable relative to each other.